Grinding wheel tester



Nov. 17, 1970 s, HAHN ETAL 3,540,269

GRINDING WHEEL TESTER Filed April 1. 1968 I 3 Sheets-Sheet 1 L /5 22 2s 26 2s FIG. 1

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F/L TER XY, Y2 PLOTTER FILTER CONTROL PANEL LOG. CONV C ILLATOR POWER AMPL/F/ER 28 FEE AMPLIFIER INVENTOR. ROBERT 5. HAHN B ROBERT 1.. PRICE Nov. 17, 1970 HA N ETYAL 3,540,269

GRINDING WHEEL TESTER Filed April 1, 1968 r 3 Sheets-Sheet 2 FIG. 3

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GRINDING WHEEL TESTER 3 Sheets-Sheet 5 I; :1 48 AZ FIG, 5 47 l 1' f I} :1 43 45 /49 'i "a |I I n: 1:! I I '14:

United States Patent O 3,540,269 GRINDING WHEEL TESTER Robert S. Hahn, Northboro, and Robert L. Price, Paxton,

Mass., assignors to The Heald Machine Company,

Worcester, Mass., a corporation of Delaware Filed Apr. 1, 1968, Ser. No. 717,852 Int. Cl. G01n 3/00 US. Cl. 73-78 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a grinding wheel tester and, more particularly, to apparatus for determining the relative hardness of abrasive wheels from wheel to wheel as well as the local variations from point to point in a particular wheel.

BACKGROUND OF THE INVENTION Users of grinding machines, particularly those in the bearing industry, today are demanding, more than ever before, machines which will grind parts to extremely close tolerances. These requirements include size, taper and roundness tolerances. They also include the ability to produce surfaces with very low micro-finish readings and surfaces with the elimination of visual surface defects. It is, therefore, important to maintain wheel u-niformity in order to produce high quality ground finishes. At the present time, the hardness of abrasive wheels varies from wheel to wheel (even when they are of the same grade of the same manufacturer) and also vary from one part to another in the same wheel. When an operation has been set up on a production grinding machine, the ability to replace a worn-out wheel with a new one without, at the same time, changing the carefully set-up machine depends on the successive abrasive wheels being of the same hardness. Furthermore, local variations in an abrasive wheel cause chatter during grinding, cause entirely different Talyrond readings at different locations longitudinally of the wheel, and cause a poor appearance in the finished workpiece surface.

Another common problem is that the vibrations set up by local hardness variations in the wheel tend to increase as the particular wheel is used; the non-homogeneous structure of, the wheel lends to uneven wheel wear and produces variations in the grinding force. When these force pulsations occur in a direction normal to the wheel and work surfaces at the line of contact between the wheel and work, they usually cause an uneven wear of the wheel surface with soft grinding wheels and uneven loading of the wheel with harder grade wheels. Because of repeated rotation of the grinding wheel, this uneven wheel wear and loading can have regenerative effects which cause the vibration to gradually increase.

Although various techniques have been used in the past to measure the hardness of abrasive wheels, none of them have been entirely satisfactory. They have been complicated, expensive, and not adaptable to use outside of the laboratory. These and other difliculties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a grinding wheel tester that is capable not only of measuring the relative hardness of successive wheels, but can determine local variations in the same Wheel.

Another object of this invention is the provision of a grinding wheel tester which is simple in construction, in-

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expensive to manufacture, and capable of a long life of useful service with a minimum of maintenance.

A further object of the present invention is the provision of a grinding wheel tester which can be used by unskilled labor to determine that all wheels in a purchased batch have average hardnesses within a predetermined range, or that a given wheel whose average hardness lies within that range does not have local hardness variations from the norm which are greater than a predetermined amount.

It is another object of the instant invention to provide a tester for determining whether or not a given abrasive wheel has local hardness variations which will cause chatter and visual marking on a finished workpiece surface.

A still further object of the invention is the provision of a tester that will permit the operator of an automatic grinding machine to determine whether a new wheel can be used without the need for re-setting the machine.

It is a further object of the invention to provide a nondestructive tester for grinding wheels.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

SUMMARY OF THE INVENTION In general, the invention consists of a grinding wheel tester having a contact member adapted to touch the surface of the wheel, having a driver for introducing vibration to the contact member and the wheel, and having means for measuring the resulting vibration as an indication of the hardness of the wheel.

More specifically, the driver frequency is varied over a range of frequencies to determine the resonant frequency of the system represented by the wheel, contact member, means, and driver. The driver frequency is varied and the said means generates a continous electrical signal indicative of the rate of acceleration of the system. On oc casion, the wheel may be rotated slowly while the resonant frequency is continuously measured. The means for measuring the frequency is a head containing an accelerometer which generates an electrical signal proportional to its rate of acceleration, so that the resonant frequency can be determined from the frequency at which the acceleration is the highest.

BRIEF DESCRIPTION OF THE DRAWINGS The character of the invention, however, may be best understood by reference to one of its structural forms, as illusrtated by the accompanying drawings, in which:

FIG. 1 is a somewhat diagrammatic representation of a grinding wheel tester embodying the principles of the present invention,

FIG. 2 is an end view of the tester taken on the line II-II of FIG. 1,

FIG. 3 is a graph showing the measured hardness around a typical abrasive wheel as measured by the tester,

FIG. 4 is a graph showing the variation of chatter during grinding with hardness of an abrasive wheel,

FIG. 5 is a front elevation of a modified form of the tester, and

FIG. 6 is a side view of the tester.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGS. 1 and 2 which best show the general features of the invention, the grinding wheel tester, indicated generally by the reference numeral 10, is shown in use with an abrasive wheel 11 which is mounted on a spindle 12 which, in turn, may be slowly turned by a motor 13. An elongated tubular housing 14 is provided with a cap 15 having an opening 16 lying adjacent the cylindrical surface of the wheel. Two guide rods 17 and 18 are mounted on the edge of the opening with their axes parallel to the axis of the Wheel. Between the rods lies a carbide roller 19 rotatably mounted on a shaft 21 extending through a bifurcated bracket 22. This bracket is restrained to movement longitudinally of the housing 14 by an aligning pin 23 operating in longitudinal grooves in the cap 15. The bracket 22 is fastened to the adjacent end of a head 24 which is capable of continuously measuring the acceleration of the system; such a head is the Wilcoxon Z602 impedance head. The head is provided with a piston 25 which is slidable in a bore 26 in the housing and which can be actuated by air pressure from a conduit 20. Fastened to the other end of the head 24 is a driver 27, similar to the Wilcoxon F-l driver, which receives electrical energy and converts it to a mechanical vibration of a desired frequency. This electrical energy arrives at the driver from a power amplifier 28 through a cable 29.

The transducer in the interior of the head 24 is connected by a cable 31 to a pre-amplifier 32. The electrical circuitry serves to convert the signal from the head into a useful signal which may be observed on an oscilloscope or recorded in the usual way. The circuitry also provides for varying the signal from the power amplifier 28 to varying the frequency of the driver 27. This circuitry includes a logarithmic converter 33, two filters 34 and 35, a logarithmic converter 36, a sweep oscillator 37, a phase meter 38, a control panel 39 and a plotter 41, all suitably interconnected.

The operation of the tester will now be readily under stood in view of the above description. To begin with, tests have shown that the relationship illustrated in FIG. 4 is valid; that is to say, there is a direct straight-line relationship between the amplitude of chatter in a grinding wheel and the stiffness variation. This last factor is the expression used to describe the amount of difference between the extremes of hardness in a wheel. In order to visualize what is happening, assume that the stiffness of the surface of the wheel increases in one particular spot. This could be compared to a log with a hard knot on the surface. When such a hard spot on the wheel comes in contact with the workpiece, the Wheel and the spindle will be deflected away from the workpiece slightly. Likewise, a deflection toward the workpiece would occur if a less stiff area of the wheel contacts the workpiece. The concept of vibration caused by stiffness variations in the surface of the grinding wheel is called parametric excitation where one of the coeflicients of the linear differential equation of motion for the vibrating system is a function of time.

Let us assume that the spindle 12 and the wheel 11 to be tested are not rotating. The driver 27 sets up a longitudinal vibration and the roller 19 is pressed against the surface of the wheel by air pressure operating on the piston 25. The local portion of the wheel can be visualized as being similar to a very stiff spring in a mechanicallyvibrating system; such a system is made up of the wheel 11, the roller 19, the head 24, and the driver 27. The frequency of the driver vibration is automatically varied over a wide range until the natural frequency of the system is discovered. This natural frequency is directly related to the local spring effect of the wheel and is indicative of the local hardness. This is because grinding wheel hardness is actually a measure of the bond strength holding the individual grains of abrasive together in the wheel; tests have shown that the hardness and the stiffness are practically in direct relationship, so that the measurement of one is, in efiect, the measurement of the other. What the apparatus accomplishes is the solution of the stiifness or spring constant of a local portion of the wheel in accordance with the following equation:

where the resonant frequency f and the mass W/g of the system are known.

By rotating the wheel, a polar plot of the hardness can be made and a typical one is shown in FIG. 3. Two curves are shown, one made with a force of 21.2 newtons and the other made with a force of 40.6 newtons.

FIGS. 5 and 6 show an alternative form of the tester in which the wheel 42 is mounted on a V-block 45 which, in turn, is clamped to a base 44 by finger 43. The wheel is rotated by hand from one position to another and each test is made with the wheel fixed. A flexible cantilever beam 46 is clamped at one end to a pyramid 47 mounted on the base. At its free end, the beam carries a carbide button 48. A preload rod 49 to which weights of various sizes can be hung extends downwardly to cause the button to bear against the wheel surface. Mounted over the button is an impedance head 51 on top of which is carried a driver 52. The head is fitted with an accelerometer which measures the amplitude of vibration of the mass. Electronic equipment, not shown, is connected to the driver and the head in the same manner as shown in FIG. 1 in connection with the first-described tester 10.

Generally speaking, the circuit includes a transfer function analyzer which is used to drive the vibration exciter, as well as to monitor and record automatically the vibration amplitude and phase angle between the driving force and the acceleration. The transfer function analyzer includes a sweep oscillator which provides a variable frequency excitaiton source for driving the vibration exciter and controls the sweep of band-pass filters and the synchronism of the XY recorder. The oscillator in the preferred embodiment sweeps over the range from 700 H to 1500 H Tests show that the contact resonant frequencies lie between these limits for all abrasive wheels in general use. The purpose of the two tracking filters is to filter the transducer signals, attenuating all signals with frequencies outside the frequency band (IO-H of the tracking filters. A phase meter provides a voltage proportional to the phase angle between the two signals under test. It is known that at resonance, the phase relationship between force and acceleration at a point goes to so that the phase meter gives an accurate determination of the resonant frequency. A XY Y plotter records the amplitude of either of two input signals or the ratio of two signals, depending upon the setting of a function selector switch on a master control panel. A logarithmic voltmeter converter changes the input signals to logarithms so that the ratios of the two signals, if desired, can be obtained by subtraction. A power amplifier is used with the vibration driver and an accelerometer amplifier is used to increase the vibration signal. The force reference signal is obtained from the oscillator output, since the voltage is proportional to the driving force and is in phase with it.

It can be seen that the use of the tester of the invention permits the application of the contact resonance method to abrasive wheels for practical rapid testing. The tester can determine not only whether the particular wheel has a hardness within a predetermined range, but also Whether it has extremes of local hardness variation which would lead to chatter, local roughness variation in the workpiece, and mottled appearance. The equipment can be used by an abrasive wheel manufacturer to maintain quality control and to grade his wheels within much closer limits than has been possible heretofore. The equipment can be used by the organization using the wheel to segregate wheels more closely than the manufacturer Wishes to and, also, to check the purchases from the wheel manufacturer. Furthermore, the wheel user may have a particular grinding operation in which automatic cycling and the like may require closer supervision of the wheel hardness than normal in that plant. That is to say, it may be advantageous for the user to buy his wheels in accordance with the wheel manufacturers grading system (since the large percentage of his work does not require any closer grading), but to set up a tester at one particular machine whose grinding operations require close control of hardness in the wheels used.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

1. A grinding wheel tester, comprising (a) a contact member adapted to touch the surface of the wheel, the contact member including a carbide roller located between two cylindrical locating rollers,

(b) a driver for introducing a vibration to the contact member and the wheel, and

(c) means varying the frequency of the vibration and observing resonance for an indication of the hardness of the wheel.

2. A grinding wheel tester, comprising (a) a contact member adapted to touch the surface of the wheel, the contact member being a carbide button mounted on the free end of a flexible cantilever beam,

(b) a driver for introducing a vibration to the contact member and the wheel, and

(c) means varying the frequency of the vibration and observing resonance for an indication of the hardness of the wheel.

3. A grinding wheel tester, comprising (a) a contact member adapted to touch the surface of the wheel,

(b) a driver for introducing a vibration to the contact member and the Wheel, and

(c) means varying the frequency of the vibration and observing resonance for an indication of the hardness of the wheel, the said means consisting of a head containing an accelerometer which generates an electrical signal proportional to its rate of acceleration, so that the resonant frequency of the system can be determined.

4. A grinding wheel tester, comprising (a) a contact member adapted to touch the surface of the wheel,

(b) a driver for introducing a vibration to the contact member and the wheel, and

(c) means varying the frequency of the vibration and observing resonance for an indication of the hardness of the wheel, the driver frequency being varied and the said means generating a continuous electrical signal indicative of the phase angle between the driver force and the acceleration of the system.

5. A grinding wheel tester, comprising (a) a contact member adapted to touch the surface of the wheel,

(b) a driver for introducing a vibration to the contact member and the wheel,

(c) means varying the frequency of the vibration,

(d) means rotating the grinding wheel, and

(e) means continuously observing resonance for an indication of the hardness of the wheel.

References Cited UNITED STATES PATENTS 2,436,435 2/1948 Kent 7385 3,116,632 1/1964 Lane 73-78 3,147,614 9/1964 Scott 7381 3,153,338 10/1964 Kleesattel 73-78 X 35 JERRY W. MYRACLE, Primary Examiner 

